The liquid-crystal display (LCD) devices typically used in prior art are for example TN (twisted nematic) LCDs, for example in accordance with Schadt, M. and Helfrich, W. Appl. Phys. Lett. 18, pp. 127 ff (1974) and in particular in their special form with low optical retardation d·Δn in the range from 150 nm to 600 nm in accordance with DE 30 22 818, STN (super twisted nematic) LCDs, such as, for example, in accordance with GB 2.123.163, Waters, C. M., Brimmel, V, and Raynes, E. Proc. 3rd Int. Display Research Conference, Kobe 1983, pp. 396 ff and Proc. SID 25/4, pp. 261 ff, 1984, Scheffer, T. J. and Nehring, J. Appl. Phys. Lett. 45, pp. 1021 ff, 1984 and J. Appl. Phys. 58, pp. 3022 ff, 1985, DE 34 31 871, DE 36 08 911 and EP 0 260 450, IPS (in-plane switching) LCDs, as described, for example, in DE 40 00 451 and EP 0 588 568, and VA or VAN (vertically aligned nematic) LCDs, as described, for example, in Tanaka, Y. et al. Taniguchi, Y., Sasaki, T., Takeda, A., Koibe, Y., and Okamoto, K. SID 99 Digest pp. 206 ff (1999), Koma, N., Noritake, K., Kawabe, M., and Yoneda, K., International Display Workshop (IDW) '97 pp. 789 ff (1997) and Kim, K. H., Lee, K., Park, S. B., Song, J. K., Kim, S., and Suk, J. H., Asia Display 98, pp. 383 ff, (1998).
In LCD devices which were known hitherto and are for the most part already commercially available, the optical appearance is inadequate, at least for demanding applications. In particular the contrast, especially in the case of coloured displays, the brightness, the colour saturation and the viewing-angle dependence of these parameters are in clear need of improvement and have to be improved if the display devices are to compete with the performance features of the widespread CRTs (cathode ray tubes). Further disadvantages of the LCD devices of prior art are often their poor spatial resolution and inadequate response times, in particular in the case of STN, but also in the case of TN or IPS and VA LCDs, in the case of the latter especially if they are to be used for the reproduction of video, such as, for example, in multimedia applications on computer display screens or in the case of television sets. Particularly for this purpose, but also even for the display of rapid cursor movements, short response times are desired.
Recently a new type of LCDs and LC devices has been reported in prior art. These devices are disclosed for example in WO 02/93244 A1, DE 10217273 A1, DE 102 41 301 or DE 103 139 79, and are hereinafter also referred to as ISP (isotropic switching panel) mode or device. They comprise an electro-optical light modulating element containing an electrically switchable mesogenic or liquid crystal (LC) medium that is operated in an optically isotropic phase, like for example in the isotropic phase (i.e. at a temperature above the clearing point of the LC medium) or, more preferably, in a blue phase. The mesogenic or LC medium in the optically isotropic phase becomes birefringent when an electric field is applied. Interdigitated electrodes on one side of the light modulation element create an in-plane electric field parallel to the plane of the element, which aligns the mesogenic or LC molecules in a planar texture along the electric field lines. A typical light modulation element of this type is schematically illustrated in FIG. 1, wherein 11 is a layer of an LC medium, 12 and 13 are linear polarisers, 14 are the electrodes and 15 depicts the electric field lines. FIG. 1A shows the element in its driven, light state where the LC material is in its liquid crystal phase. FIG. 1B shows the element in its undriven, dark state, where the LC material is in an optically isotropic phase, preferably in a blue phase. The field-induced birefringence of the LC layer retards the incident linearly polarised light, enabling it to pass through the second polariser. The optics of the light state can be approximated to that of a planar aligned LC between crossed polarisers. Although the regions directly above the electrodes are homeotropic, the majority of the LC is planar aligned. The optics of the dark state is that of an optically isotropic medium between crossed polarisers.
ISP LCDs, details on their assembly and suitable components and LC media to be used therein are described in WO 02/93244 A1, DE 10217273 A1, DE 102 41 301 and DE 103 139 79, the entire disclosure of which is incorporated into this application by reference.
ISP LCDs are suitable, inter alia, as display screens of television sets, computers, such as, for example, notebook computers or desktop computers, central control units and of other equipment, for example gambling machines, electro-optical displays, such as displays of watches, pocket calculators, electronic (pocket) games, portable data banks, such as PDAs (personal digital assistants) or of mobile telephones.
However, LCDs of the ISP mode often show a limited viewing angle performance. For example, in a typical ISP mode display as shown in FIG. 1, in specific directions the dark state often exhibits light leakage which results in contrast reduction. In addition, the light state luminance is often reduced and colouration can occur. Also, the contrast is often limited which is a disadvantage especially in large area applications such as television.
The inventors of the present invention have found that the properties of ISP LCDs, especially the viewing angle characteristics, can be further improved by using a compensator comprising one or more retardation films of specific types and in specific arrangements as described in the following invention. In particular, this invention relates to optical films which can be applied to ISP LCDs to improve the viewing angle in terms of dark state luminance (and hence contrast) and surprisingly, the white-state luminance and colour.
One aim of the present invention is to provide a compensator for a light modulation element or display of the ISP mode with improved optical performance. The compensator should preferably be easy to manufacture and allow economic fabrication even at large scales.
Another aim of this invention is to provide an advantageous use of the compensator according to this invention.
Another aim of this invention is to provide a light modulation element or display of the ISP mode comprising an inventive compensator which shows advantageous properties such as good contrast, reduced colour shift and wide viewing angles.
Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
The above aims can be achieved by providing compensators, light modulation elements and displays according to the present invention as described above and below.